1. Field of the Invention
The present invention relates generally to a single-phase three-wire type transformer and, more particularly, to a single-phase three-wire type transformer in which a secondary coil is divided into a plurality of coils to be arranged in a core so that these coils are connected in an intersected condition in order to avoid an imbalance in the secondary voltage.
2. Description of the Background Art
Some single-phase three-wire type transformers have a structure so that a secondary coil is divided into a plurality of coils to avoid an imbalance in secondary voltages (due to a connection state of loads) to be arranged in a core so that these coils are connected in an intersected condition. Such single-phase three-wire type transformers are referred to as division intersection connections and generally have been widely used.
In other words, a single-phase three-wire type transformer adopting the division intersection connection, as shown in FIG. 4, includes a core 1 of an iron frame of an approximately square configuration, and conductors are wound opposite on two locations on the core 1, respectively, to form a coil A and coil B. However, these coils A and B are not merely an independent primary or secondary coil, respectively, but make up three-layer structures with three overlapped and wound coils, respectively, as shown in FIG. 5. The coil A is constituted so that secondary coils 21a and 22a and a primary coil 11a are overlapped and wound in sequence from the inside of the core 1. The coil B is similarly constituted so that secondary coils 21b and 22b and a primary coil 11b are overlapped and wound in sequence from the inside of the core 1. These connections are made so that the primary coils 11a and 11b are combined in series with the respective other ends of the coils to be set as primary terminals 1a and 1b in the primary coils. The secondary coils 21a and 22b are combined at a connection point 2x and the secondary coils 22a and 21b are connected at a connection point 2y to cause the connections to be intersected. Then, the other ends of the secondary coils 22a and 22b are combined to make this connection point a secondary terminal 2n, and the other end of the secondary coil 21a is made a secondary terminal 2u and also the other end of the secondary coil 21b is made a secondary terminal 2v.
When the connections are intersected in this way, when a load is connected only between the secondary terminals 2u and 2n, for example, an electric current will flow from the secondary terminal 2u through the secondary coils 21a and 22b to the secondary terminal 2n, so that an electric current can flow through both the coils A and B to maintain the balance of magnetic flux for the core 1, resulting in equilibrium of the voltage.
In addition, in order to increase the electric current capacity in the secondary coils, it is necessary to adopt a thick winding conductor with an increased cross-sectional area for the winding conductor of the secondary coils 21a, 22a, 21b, and 22b. However, when the diameter of the winding conductor is made large, there may arise a disadvantage in which eddy current loss may become large, causing the conversion efficiency of the transformer to be decreased. Therefore, each secondary coil is made double by winding two parallel winding conductors of small diameter on the core 1, and secondary coils are constituted by connecting each doubled secondary coil in an intersecting condition. That is, as shown in FIG. 6, the secondary coil 21a has a duplex structure of coils 211a and 212a made by winding two parallel winding conductors of small diameter. Similarly, the secondary coils 22a, 21b, and 22b have a duplex structure of coils 221a and 222a, coils 211b and 212b, and coils 221b and 222b, respectively. Furthermore, these duplex coils are connected in parallel by combining the respective lead portions extending from the ends of the duplex coils. As for the combinations between the coils, as discussed hereinbefore, the secondary coils 21a and 22b are combined at the connection point 2x and the secondary coils 22a and 21b are connected at the connection point 2y causing the connections to be intersected. Then, the other ends of the secondary coils 22a and 22b are combined to make this connection point to be the secondary terminal 2n, and another end of the secondary coil 21a is made the secondary terminal 2u, and the other end of the secondary coil 21a is made the secondary terminal 2v.
In this case, although the diameter of the winding conductor is small, each secondary coil has a duplex structure, so that the electric current capacity is increased substantially to double that of a conductor with a small diameter, and because the diameter of the winding conductor is small, the eddy current loss can be suppressed to a low level.
However, a single-phase three-wire type transformer of the prior art described above has a disadvantage inasmuch as when each secondary coil is configured with a duplex structure, four closed circuits are formed among the secondary terminals 2n, 2u, and 2v and connection points 2x and 2y of the intersection connections so that circulating currents according to electromotive forces originating from the distribution of magnetic flux density may flow through these closed circuits, resulting in a loss W.
That is, among the secondary terminals 2n, 2u, and 2v and connection points 2x and 2y of the intersection connections, there are formed a closed circuit C1 with a current circulating through the secondary terminal 2u, coil 211a, connection point 2x, coil 212a, and the secondary terminal 2u, a closed circuit C2 with a current circulating through the secondary terminal 2n, coil 222b, connection point 2v, coil 212b, and the secondary terminal 2n, a closed circuit C3 with a current circulating through the secondary terminal 2v, coil 212b, connection point 2y, coil 211b, and the secondary terminal 2v, and a closed circuit C4 with a current circulating through the secondary terminal 2n, coil 221a, connection point 2y, coil 222a, and the secondary terminal 2n.
Furthermore, there is, of course, a magnetic field (a leakage magnetic flux) outside the core 1 in this transformer. The distribution of the magnetic flux density will be described using FIG. 2 according to the present invention. The magnetic flux density reaches a peak value on an interface of the primary and secondary coils, as shown in FIG. 2, and the electromotive force (V) is generated in proportion to this magnetic flux density (B), so that the circulating current flows in each closed circuit. When the peak value of the electromotive force is assumed to be V, as the secondary coils 21a and 22a are composed of four layers, so the respective electromotive forces among each of the layers become (1/4)V between layers 1 and 2, (2/4)V between layers 2 and 3, and (3/4)V between layers 3 and 4. Similarly, as the secondary coils 21b and 22b are composed of four layers, so the respective electromotive forces among each of the layers become (1/4)V between layers 1 and 2, (2/4)V between layers 2 and 3, and (3/4)V between layers 3 and 4.
Therefore, as shown in FIG. 7, circulating currents may flow based on the electromotive forces generated among each of the layers of the secondary coils in each of the closed circuits, and when the resistance component of each closed circuit is assumed to be R, the loss in the closed circuit C1 will become .vertline.(1/4)V.vertline..sup.2 /R, similarly, the loss in the closed circuit C2 will become .vertline.(3/4)V.vertline..sup.2 /R, the loss in the closed circuit C3 will become .vertline.(1/4)V.vertline..sup.2 /R, and the loss in the closed circuit C4 will become .vertline.(3/4)V.vertline..sup.2 /R. Therefore, the loss W in this transformer will become the sum of each loss described above, i.e., (5/4).times.(V.sup.2 /R) Incidentally, the resistance components of each closed circuit are equivalent to a resistor value generated when two coils constituting a duplex coil are connected in parallel, and the resistor value of a winding conductor itself of a coil is so small that the variation of resistor values among the coils so completed is very small. Consequently, all of the resistor values may be considered to be the same value.
The present invention has been made in view of the above-described background, and therefore, has objects to solve the above-described problems, to enable the induced magnetic flux to be balanced on the magnetic path regardless of the connection condition according to the division intersection connection, and also to enable the electric current circulating through the inside of a circuit of a transformer to be reduced even when secondary coils are formed with a duplex coil configured by winding two conductors in parallel, thereby providing a single-phase three-wire type transformer which can reduce the loss in the coils.